Typical lithographic and offset printing techniques utilize plates that are permanently patterned, and are, therefore, useful only when printing a large number of copies of the same image, such as magazines, newspapers, and the like. Variable data digital lithography or digital offset lithographic printing has been developed as a system that uses a non-patterned re-imageable surface, which is initially uniformly coated with a dampening fluid layer. Regions of the dampening fluid are removed by exposure to a focused radiation source (e.g., a laser light source) to form pockets. A temporary pattern in the dampening fluid is thereby formed over the non-patterned re-imageable surface. Ink applied thereover is retained in the pockets formed by the removal of the dampening fluid. The inked surface is then brought into contact with a substrate, such as paper, plastic or metal and the ink transfers from the pockets in the dampening fluid layer to the substrate. The dampening fluid may then be removed, a new uniform layer of dampening fluid applied to the re-imageable surface, and the process repeated.
Digital offset printing systems use offset-type inks that are specifically designed and optimized to be compatible with the materials the ink is in contact with, including the re-imageable surface and the dampening solution as well as with the various subsystems used during the printing process to enable high quality digital printing at high speed.
For example, an inker subsystem may be used to apply a uniform layer of ink over the layer of dampening fluid. The inker subsystem may use an anilox roller to meter the ink onto one or more ink forming rollers that are in contact with the re-imageable surface. The ink used with this subsystem should have a viscosity that is not so high that anilox-take up and delivery to the re-imageable surface is difficult. However, too low of a viscosity, tack and/or poor cohesion may result in the ink crawling out of the ink loader, resulting in unwanted spills, loss of ink and potential contamination of the printer. Accordingly, digital offset inks should have a certain range of viscosity, tack and tack stability to afford sufficient and predictable ink cohesion to enable good transfer properties in and among the various subsystems.
Although there is a growing demand to expect digital offset printing to produce white for background labels, tinting purposes or special effects on metallic substrates, for example, many white inks known in the art do not have the necessary transfer properties. Previous formulated white inks containing titanium dioxide (TiO2) pigment, for example, have relatively low tack and poor ink cohesion or higher tack but poorer tack stability over time.
Further, the hiding power (the ability to effectively mask an underlying color) of a white ink image on a substrate is associated with brightness and reflection properties, which are extremely sensitive to the amount of white ink that is transferred. Reduced tack and/or reduced tack stability may thus lead to insufficient coverage of substrates printed using digital offset printing architecture.
Accordingly, there is a desire in the art for white inks that meet all of the functional requirements of the sub-systems for digital offset printing while retaining sufficient ink tack stability to provide adequate coverage of white ink on a printed substrate.